There are many contexts in-which it is desired to ascertain the presence of a concealed object. In medical practice particularly, and even more particularly in surgical practice, there is a critical need to track the locations of medical tools, devices, aids, materials, and other objects. And the most critical need of all is to ensure that no such medical objects are unintentionally left behind inside a patient's body cavity after a surgical procedure has been completed.
For providing best estimates of the position or location of an object, detectors adapted for sensing the object are preferably fixed in space at known positions in the space. To most closely realize this goal when using a movable detector, the detector is preferably moved automatically and repeatably by robotic devices to ensure that measurements correspond to specific points in space. However, in the medical theater, there are typically a number of lines and wires connecting the patient to various devices and implements that are present in the vicinity, there may be a number of surgeons and other personnel standing in the vicinity, and there is an abundance of equipment, trays and carts for holding tools in proximity to this personnel, so that the area around the patient's body is cramped and difficult to negotiate. For these reasons as well as others, it has been found to be impractical to use fixed sensing devices, or a sensing device that is moved on, e.g., a track, or with a pre-programmed robotic arm, for sensing medical objects inside a patient.
Position sensing using hand-held devices is known in medical imaging, and position tracking has been used to track the movements of surgical instruments. For example, Sliwa Jr., et al., U.S. Pat. No. 6,122,538 (“the '538 Patent”), refers to a movable ultrasound transducer probe utilizing a set of translational accelerometers for measuring both translational and angular accelerations. Position and angle changes are deduced by double integration. However, the '538 Patent acknowledges that the system is subject to substantial inaccuracies, making it difficult or impossible to return to a previous scan position. Hence, the '538 Patent proposes a hybrid system-that also incorporates a gyroscopic sensor array having three orthogonal gyroscopes. This type of device is commonly referred to in the art as an “inertial guidance system,” although the system need not be used for guidance and can be used, as suggested in the above reference, merely for sensing.
Ferre et al., U.S. Pat. No. 6,687,531 (“the '531 Patent”), provides an example of tracking the movements of a surgical instrument. In one embodiment, the location of the instrument may be determined with respect to fixed transmitters (or receivers) attached to the patient's head by triangulation. In another embodiment, a field is generated using three orthogonally disposed magnetic dipoles, and the known near-field characteristics are used for position detection.
Neither of the aforementioned systems is suited to searching for, or detecting the location of, concealed and unknown objects. The '538 Patent merely detects the location of a probe that is in the possession of a user of the system. The probe is not used to acquire data about another object having unknown characteristics. The '531 Patent merely tracks the location of a probe as it moves in the body. The standard prior art methodologies for tracking the location of surgical instruments or other medical objects left in a patient's body are either to X-ray the patient, which is undesirable for obvious reasons, or simply to count the objects used during the surgery before and after the surgery.
Some more sophisticated approaches for detecting medical objects in a patient's body cavity have been proposed. These fall into two basic categories, depending on the constitution of the medical object. On the one hand, metal medical objects can be detected using, essentially, a metal detector. Avrin et al., U.S. Pat. No. 5,842,986 (“the '986 Patent”), is exemplary. On the other hand, some medical objects, such as sponges, do not interact with electromagnetic fields. However, such objects can be tagged with RFID (acronym for “radio frequency identification”) tags and the tags can be stimulated to produce an output that can be detected. Blair et al., U.S. Pat. No. 6,026,818 (“the '818 Patent”), is exemplary of this approach.
Hobbyists use metal detectors for discerning the presence of buried coins and other artifacts, and metal detectors are also used, now ubiquitously, in security systems for discerning the presence of concealed weapons. Metal detectors may sense radiation reflected or otherwise transmitted from the object, or may sense a change in the inductance of a coil that is induced by the presence of a nearby metal object. The '986 Patent in particular proposes applying a low strength, time varying magnetic field to a screening region of a body and sensing a responsive magnetic field from a retained ferrous body within the screening region. As in standard metal detection practice, only the component of the responsive field that oscillates at the frequency of excitation is used, to minimize the effect of background noise. However, unlike the standard practice, the location of the source of the responsive field is inferred from the response at a single location in space by measuring field gradients. Further, to increase detector sensitivity over standard metal detection techniques, the '986 Patent proposes the use of an improved magnetorestrictive sensor.
For detecting non-metallic medical objects, the '818 Patent proposes detecting the output of an RFID tag that has been attached to the medical object. Such tags are attached to medical objects by the manufacturer. The tags produce a coded output when stimulated by a time-varying electromagnetic field at the proper frequency. The output is a weak, narrow band signal that is modulated to identify a particular product/manufacturer combination.
As the '818 Patent explains, there is a problem obtaining sufficient signal strength from a small tag. However, increasing the size of the tag to provide greater signal strength is problematic for surgical sponges, which are asserted to be the most common medical object for which detection is important, because deformation of the sponge will often deform the tag, and using a large, non-deformable tag would defeat the purpose of the sponge.
The proposed solution to this problem is to increase the sensitivity of the detector so that it is capable of detecting the weak signals produced by small, inexpensive tags by stimulating the tag with a pulsed signal which covers a wide frequency range that includes the frequency of the tag. The pulsed signal triggers a continuing response signal from the tag, at its single frequency, which increases to a point where it becomes differentiated from background noise.
Both the '986 and the '818 Patents propose useful and desirable improvements to increasing detector sensitivity, for detecting the presence of and, to some extent, for locating the respective classes of medical objects within a body cavity in the operating room environment. However, the '986 patent does not recognize that the increased sensitivity of its detector will be accompanied by an increased sensitivity to noise, and that the standard practice of measuring only components of the responsive field that are synchronized to the frequency of excitation will not provide any additional noise reduction than has always been available in metal detectors, and which is known to be insufficient for this medical purpose.
Moreover, neither the '986 nor the '818 Patents recognize that the operating room is filled with other, similar medical objects, as well as many sources of general, broad-band electromagnetic radiation (such as CRT's), that are not located inside the patient but that are nevertheless nearby and will be detected. Many of the medical objects left behind inside a patient, such as needles and tagged sponges, will produce small signals or will only weakly interact with an interrogating field. Many of the external objects and sources will produce a stronger signal or other indication of presence, and neither patent provides any guidance for ensuring that what is detected is in fact of interest. Moreover, the improvements proposed in these patents are specific to sensing particular classes or types of objects; neither patent provides any guidance for improving the accuracy, precision, or specificity of detection and location in general, independent of what is being detected and the sensor technology being used.
Therefore, as the present inventor has recognized, there is a need for a method and apparatus for collecting data for detecting and locating disturbances that has particular applicability in the context of searching for medical objects, such method and apparatus also having wider applicability as discussed hereinbelow.